Make sure widget manager is not null in MenuWidget.hide() (#3104)

Fixes #3092

Author: bluemarvin

app/src/common/shared/org/mozilla/vrbrowser/ui/widgets/menus/MenuWidget.java

@@ -65,14 +65,18 @@ public void updateUI() {
     public void show(@ShowFlags int aShowFlags) {
         super.show(aShowFlags);
 
-        mWidgetManager.addFocusChangeListener(this);
+        if (mWidgetManager != null) {
+            mWidgetManager.addFocusChangeListener(this);
+        }
     }
 
     @Override
     public void hide(@HideFlags int aHideFlags) {
         super.hide(aHideFlags);
 
-        mWidgetManager.removeFocusChangeListener(this);
+        if (mWidgetManager != null) {
+            mWidgetManager.removeFocusChangeListener(this);
+        }
     }
 
     public void updateMenuItems(ArrayList<MenuItem> aItems) {